Continuous casting is a process in which molten metal is cooled and made to solidify in order to continuously supply a molded piece in a predetermined shape.
FIG. 1 shows a continuous casting machine used for continuous casting process. The continuous casting machine includes a ladle vessel 510, a long nozzle 520, a tundish 530, a plurality of immersion nozzles 150 and a plurality of molds 540. FIG. 1 shows a single immersion nozzle and a single mold alone.
Molten metal, for example molten steel, fed to the ladle vessel 510 is delivered through the long nozzle 520 to the tundish 530. Molten metal stored in the tundish 530 is infused through the plurality of immersion nozzles 150 to the plurality of molds 540. The molds are water-cooled and the infused molten metal is cooled and made to solidify so that a metal piece, for example, a slab is continuously formed.
Keeping a level of molten metal in a mold of the continuous casting machine constant is important in improving product quality. Accordingly, various devices and methods have been developed to measure a level of molten metal in a mold.
A method in which an electrode is used as a path of electromagnetic wave when a level of molten metal in a mold is measured through propagation time of electromagnetic wave has been developed (JP3107183B). In this method, electrodes are inserted into the molten metal and a level displacement of the molten metal is measured using a difference in propagation time of a transmitted signal, which depends on the level displacement. More specifically, a first electrode and a second electrode are inserted into the molten metal which is electrically conductive. A first pseudo random signal is transmitted to the first electrode. A first product is obtained by multiplying the first pseudo random signal by a second pseudo random signal having the same pattern as that of the first pseudo random signal and a frequency which is slight different from that of the first pseudo random signal. A second product is obtained by multiplying a signal obtained from the second electrode by the second pseudo random signal. Then, a level displacement of the molten metal is calculated based on a time difference between a time interval of points of maximum correlation values in time pattern of the first product and a time interval of points of maximum correlation values in time pattern of the second product.
The reason that in the method described in JP3107183B electrodes are used as a path of propagation of electromagnetic wave is below. A common method in which a transmission antenna sends electromagnetic wave to the surface of the molten metal in the mold and a reception antenna receives electromagnetic wave reflected on the surface is affected by multipath reflection and cannot carry out a high accuracy measurement. Further, an antenna of a sufficiently large size can hardly be installed because of small spaces around the mold.
FIG. 2 illustrates the method for measuring a level of molten metal described in JP3107183B. In order to measure a level of molten metal (molten steel) 600 from the initial state, two electrodes 550A and 550B of length of approximately from 300 to 400 millimeters have to be inserted into the molten metal. When a metal having a melting point lower than that of the molten metal is used for the electrodes 550A and 550B, the electrodes 550A and 550B in contact with the molten metal 600 will dissolve and disappear. Accordingly, a level of the molten metal can be measured when the level of the molten metal is moving upward while it cannot be measured when it is moving down. When a metal having a melting point higher than the molten metal is used for the electrodes 550A and 550B, the electrodes 550A and 550B will be taken off by the metal (steel) 600 which has solidified after being cooled in the mold. In either case, the electrodes 550A and 550B have to be installed for each measurement. Since a space between the tundish 530 and the mold 540 are small, the two electrodes 550A and 550B have to be installed before the tundish 530 moves to a position above the mold 540. Thus, the installation requires time and labor consuming operation.
A method in which the tundish is used as a path of transmission of electromagnetic wave in place of an electrode has been developed (JP9-178533A). However, this method also employs another electrode, and therefore the above-described problem will not be solved.
Thus, an apparatus for measuring a level of molten metal using electromagnetic waves which allows high-accuracy measurement of a level of molten metal without the requirement for time and labor consuming operation has not been developed.